The invention relates to a mass flow measuring apparatus having a tube, which at a first position is coupled as regards oscillation to a first energy converter and at a second position is coupled as regards oscillation to a second energy converter, and having an evaluating device, the first and the second position being spaced from one another, the energy converters each being operable as oscillation generator and oscillation detector and the energy converters working alternately as oscillation generators. The invention relates furthermore to a method of measuring a mass flow through a tube that is caused to vibrate at a first position and at a second position alternately, signals being measured at the second and at the first position.
Such a mass flow measuring apparatus and such a method are known from DE 39 23 409 A1. The mass flow measuring apparatus described therein works in accordance with the Coriolis principle. The tube is set oscillating at the first position and the xe2x80x9cresponsexe2x80x9d at the second position is recorded. The tube is then set oscillating at the second position and the responses at the first position are recorded. From the difference between the propagation times of the waves in the one and in the other direction, conclusions can be drawn about the mass flow.
The principle has proved successful, but has the disadvantage that the wave propagation of the oscillations that have been used until now was very fast, so that the tube had to be very long in order to be able to achieve a time difference necessary for an accurate measurement.
In a paper by J. Hemp, xe2x80x9cThe Theory of Coriolis Mass Flowmetersxe2x80x9d, Cranfield Institute of Technology, Cranfield, England 29th to 30th Oct. 1987, a Coriolis mass flow measuring apparatus having a U-shaped tube is described, two energy converters being mounted on the tube. The energy converters operate alternately as oscillation generator and oscillation detector. In this case, the U-shaped tube is set permanently oscillating and oscillates at its natural frequency.
The invention is based on the problem of achieving accurate measurements even with a measuring apparatus of small construction.
That problem is solved in the case of a mass flow measuring apparatus of the kind mentioned in the introduction in that the energy converters are constructed as part of resonant circuits and the evaluating device determines a predetermined parameter of a sympathetic oscillation at the second energy converter after excitation by the first energy converter and the parameter of the sympathetic oscillation at the first energy converter after excitation by the second energy converter.
In this construction, by the use of the resonant circuits and the sympathetic oscillations associated therewith, the time available for a measurement is xe2x80x9cstretchedxe2x80x9d. It is therefore possible, even with a relatively short measuring tube, to determine satisfactory differences between oscillation propagation in the one direction and oscillation propagation in the other direction. This difference is ultimately a measure of the mass flow through the tube. In this case, the following consideration is used as a starting point: an oscillation that is coupled into the tube propagates along the tube and in so doing reaches the oscillation detector. The tube is therefore caused to oscillate also in the region of the oscillation detector. It will nevertheless take a while for the tube to reach a steady state in the region of the oscillation detector, that is, to oscillate so that the oscillation there is comparable with the excitation oscillation. It is now possible to exploit this xe2x80x9ctime gainxe2x80x9d to permit more accurate measurements. For measurement of the flow, use is made of the fact that the oscillations propagate not only via the tube but also via the liquid that is located in the tube and the mass flow of which is to be determined. The oscillation propagation can now be made in the one direction, that is, with the mass flow, and at the same time a parameter of the sympathetic oscillation can be evaluated at the oscillation detector. The oscillation can subsequently be allowed to run in the reverse direction and the sympathetic oscillation can be evaluated at the other oscillation detector. As is well known, conclusions about the mass flow can then be drawn from the propagation time differences. The corresponding differences can certainly be formed with greater accuracy because basically a longer time is available. Expressed in simple terms, the measuring principle is based on a comparison of the oscillation coupling by the tube with the oscillation coupling by the liquid in the tube.
Preferably, the tube causes a mechanical coupling between the first and the second position, in which a momentum conveyed by the tube is in the range of 10 to 1000 times greater than a momentum conveyed by the fluid in the tube. The momentum can also be called a pulse. In this way, the oscillation coupling by the liquid can be ascertained satisfactorily. It is virtually impossible to influence the oscillation coupling by the liquid, but the tube can be designed so that the said condition is fulfilled. For example, a tube of metal having a low modulus of elasticity can be used, or a tube having a thin wall. The greater is the xe2x80x9chydraulic couplingxe2x80x9d, that is, the oscillation coupling by the fluid in the tube compared with the oscillation coupling by the tube itself, the more significant are the differences when detecting the oscillation in the one and in the other direction, that is, with and against the flow through the tube.
In this connection it is preferred that coupling of the two resonant circuits lies in the region of a so-called critical coupling. In this region, the coupling is adapted so that the one resonant circuit just impacts on the other. If the coupling is overcritical, the oscillation energy will migrate back and forth between the two. If it is less than the critical coupling, there will be no impact on the resonant circuit to be excited.
Preferably, excitation is time-limited. For example, 30 measurements per minute can be taken. Subjecting the excitation to a time limit ensures that oscillation propagation in the one direction and oscillation propagation in the other direction can be determined sufficiently often.
Preferably, the parameter is a predetermined amplitude of an oscillation envelope that can be tapped off at the oscillation detector. Starting from a non-oscillating tube, after the tube has been caused to oscillate at the first position, an oscillation will develop on the resonant circuit that is arranged at the second position, the oscillation having an amplitude that increases over time until this oscillation, which is also called the xe2x80x9csympathetic oscillationxe2x80x9d, has the maximum amplitude, the maximum amplitude in turn being determined by the excitation energy at the first position. If the envelope of the sympathetic oscillation is now formed, it is observed that the amplitude of this envelope increases over time. The length of time taken until the envelope reaches a specific amplitude can now be measured, and this time can be used as propagation time substitute for the propagation of the oscillation in the one direction or in the other direction.
In this connection, the evaluating device preferably has a threshold control and a comparator. By means of the threshold control, it is possible to pre-set the threshold at which, when it is exceeded, which can be ascertained by means of the comparator, a time can be measured.
In an alternative construction, the parameter can be a phase difference between the sympathetic oscillation and another oscillation. This phase difference can be determined, for example, by monitoring passages of the corresponding oscillations through zero. In this case, it is assumed that between the, sympathetic oscillation and the excitation oscillation a phase shift has generally occurred, which can be used as criterion for the oscillation propagation and hence as measuring time for determining the mass flow. The magnitude of the phase shift is amplified by the resonant circuits used. This amplification is based on the fact that the oscillations transmitted through the fluid are out of phase by 90xc2x0 compared with the oscillations transmitted through the tube. The resonant circuit is therefore-triggered or excited by two oscillations that are out of phase. This leads to a flow-dependent phase shift of the sympathetic oscillation.
In this connection, the evaluating device preferably has at least one memory device, in which a sympathetic (i.e. resonant) oscillation or a part thereof can be stored. The sympathetic oscillation is then available in its time characteristic for subsequent evaluation.
In this connection, a comparator is preferably provided, which compares an actual sympathetic oscillation with an exciting signal or with the stored sympathetic oscillation. Direct comparison of two oscillations enables a phase angle between the two oscillations to be established relatively accurately, whereby it is possible to determine the mass flow with great accuracy. The actual sympathetic oscillation also need not necessarily be evaluated at the instant of its appearance. It too can be sampled and stored.
The resonant circuits preferably comprise multi-mass oscillators, an outer mass being constructed as the largest mass. By this means, the oscillations on the tube have a preferred direction. The largest mass of each oscillation generator serves in this connection as xe2x80x9cisolatorxe2x80x9d, even though oscillations will naturally be observed also on the other side of the largest mass.
Between the first position and the second position, a third resonant circuit is preferably coupled as regards oscillation with the tube. A further delay in or stretching of the measuring time can therefore be achieved. An oscillation of the tube and the liquid located therein that is generated at the first position sets the tube and the fluid located therein oscillating first of all at the third resonant circuit, a transient phenomenon also being observed here. The oscillation will not propagate to the energy converter at the second position until the third resonant circuit has reached maximum oscillation; it is, nevertheless, to be observed that development of the sympathetic oscillation at the second position takes longer when a third resonant circuit is arranged in the path between the first and the second position.
In a method of the kind mentioned in the introduction, the problem is solved in that the signals are in The form of sympathetic oscillations and a parameter of the sympathetic oscillation at the first position, and the same parameter of the sympathetic oscillation at the second position are used to determined the flow.
As stated above, more time or a more accurate resolution is available for determining the propagation times between the first and the second positions or between the second and the first positions, that is, in the direction of flow and in the direction against the flow. The tube can therefore be kept short, without having to sacrifice measuring accuracy.
The amplitude of an envelope of the sympathetic oscillation is preferably used as parameter. The measure of how quickly the sympathetic oscillation builds up is a measure of the mechanical coupling between the first and the second positions and between the second and the first positions. The mechanical coupling is based on the one hand on the oscillation coupling by the tube and on the other hand on the oscillation coupling by the liquid. The oscillation coupling by the liquid is dependent on the direction of flow and the mass flow through the tube, however. By comparing the measurements in the direction of flow and in the direction against the flow, sufficient information can be gained to obtain the mass flow through the tube.
Alternatively, a phase difference between the actual sympathetic oscillation and the excitation oscillation or a stored, previous sympathetic oscillation can be used as parameter. Such a phase difference can be determined relatively accurately when both oscillations are, as it were, available at the same time.
Preferably, an excitation circuit frequency of the order of magnitude of       ω    ex    =                    E        ·        I                                          k            1                    ·                      ρ            tube                          +                              k            2                    ·                      ρ            liquid                              
is chosen, in which xcfx89ex is the excitation circuit frequency, xcfx81tube and xcfx81liquid are the density of the tube and liquid respectively and k1 and k2 are constants that depend on the flow measuring apparatus, E is the modulus of elasticity of the tube and I is the bending moment of inertia of the tube. In this way, the best possible transfer between the oscillation exciter and the oscillation detector is obtained, so that the loss of information remains relatively small. In this connection, it is not absolutely necessary to meet the above-mentioned excitation circuit frequency exactly, it the circuit frequency is met at least in respect of order of magnitude. The excitation frequency should lie within the bandwidth of the resonant circuits.